1. Field of the Invention
This invention relates to a fluid-flow transducer for producing a pressure differential responsive to rate of at least normally unidirectional fluid flow.
More particularly, this invention relates to a fluid-flow transducer of a type for producing a pressure differential responsive to rate of at least normally unidirectional fluid flow, comprising: a housing forming a flow passage for the fluid; said passage having formed therein a throat which diverges progressively in the normally downstream direction; a valve member adapted and arranged for obturating the smaller end of the throat when there is no flow or very little flow; the valve member being carried by a spindle which extends in the normally upstream direction from the throat and is slidably supported by a bearing upstream of but adjacent the throat, the bearing being fixed to the housing; and an axially extending compression coil spring for biasing the valve member in the upstream direction relative to the housing to tend to close the throat; the housing having two ports therein, respectively upstream and downstream of the throat, for detection of a pressure differential induced across the throat by a fluid flow.
2. Description of the Prior Art
British Patent Specification No. 1 490 989 discloses such a fluid-flow transducer in FIG. 1 of the drawings of British Patent Specification No. 1 490 989. However, this transducer exhibits a number of defects or disadvantages. Firstly, the "housing" is a pipe-line with (presumably) its upstream and downstream ends remote from the throat 25, so it is not clear how the throat would be fabricated, particularly since the throat is not a simple frusto-conical throat but has a complex curvature. Secondly, the compression spring which biases the valve member in the upstream direction is located downstream of the valve member itself and it is not clear how the downstream end of the compression spring would be supported, other than by a support (not disclosed) having the effect of severely restricting the flow passage. Thirdly, the bearing only engages a relatively short axial length of the spindle, which is therefore liable to wobble. This liability to wobble is especially disadvantageous for very small rates of flow when the valve member is obturating, or almost obturating, the smaller end of the throat, since it will result in relatively large variation in the gap between the valve member and the smaller end of the throat through which the fluid flows, with consequential variation in the pressure differential produced by the fluid flow, when the rate of flow is very small.
Furthermore, British Patent Specification No. 1 490 989 does not suggests how accurate concentricity is to be achieved between the valve member and the smaller end of the throat.
In FIG. 3 of British Patent Specification No. 1 490 989, there is proposed a bi-directional construction, to permit reverse fluid flow. However, the proposed construction is impractical for a number of reasons. Firstly, the bearing is spaced upstream of the smaller end of the throat, so that there is a large overhang of the spindle, with consequential inaccurate location of the valve member at the place where it is required to be most accurately located, namely, at the narrowest part of the throat (either throat, that is). Secondly, there is no suggestion as to how to adjust the initial or zero flow position of the valve member, and no obvious way of doing so.
Other, less relevant, prior art includes British Patent Specification Nos. 1 190 912 and 1 566 251 (both Gervase Instruments Limited) and U.S. Pat. No. 3 554 031 (D. Turner).